When a measurement target gas which is to be measured by a thermal flow meter flows in a state of a laminar flow only in a forward direction, it is easy to measure the flow rate of the measurement target gas. However, such case is rare. In many cases, not only a flow in the forward direction but also a flow in the backward direction may occur in the measurement target gas, and in addition, a vortex may be generated.
For example, when a thermal flow meter is provided in a vehicle to be used to measure the flow rate of intake air guided into an internal combustion engine, the intake air which is the measurement target gas flows in a state of pulsing motion based on the operation of the internal combustion engine, and further, in a particular operation range of the internal combustion engine, a backward flow phenomenon occurs in a mixed manner, in which the intake air flows not only in the forward direction but also in the backward direction. In order to measure the amount of the intake air acquired into the internal combustion engine in the intake process of the internal combustion engine, it is desired to accurately measure not only the flow rate of the intake air flowing in the forward direction but also the flow rate of the intake air flowing in the backward flow, and derive the amount of the intake air actually acquired into the internal combustion engine from the flow rate in the forward direction and the flow rate in the backward direction. For this reason, the thermal flow meter is desired to be able to measure not only the flow rate in the forward direction but also the flow rate in the backward direction. An example of such a thermal flow meter capable of measuring both of the flow rates in the forward direction and in the backward direction is described in PTL 1.